In the parent applications, a unique technique for enhancing kraft cooking is provided utilizing one or more circulation-dilution loops in addition to conventional extraction and dilution mechanisms, and by reintroducing liquor having lower dissolved organics (such as dissolved cellulose, lignin and hemicellulose) than the withdrawn liquor. It has now been found, according to the present invention, that the same basic technique of additional circulation-dilution loops can be utilized to perform other worthwhile functions. In particular, according to the present it has been found that a method--utilizing the additional circulation/dilution loops--for selectively increasing the sulfidity and sulfide ion concentration of the kraft cooking liquor (e.g. white liquor) during kraft cooking of comminuted cellulose fibrous material (wood chips) may be provided, which is especially advantageous at or near the impregnation and/or first cooking zones. It has also been found according to the present invention that such additional circulation-dilution loops can be utilized to maintain the pressure in the digester at a desired superatmospheric level (e.g., the conventional level of about 165 psi) or maintain the liquor level in a manner that avoids non-uniform unstable material movement in the countercurrent washing zone, and--depending upon the particulars of the method--anywhere in the digester.
The active cooking chemicals in kraft cooking liquor, e.g. white liquor, are sodium hydroxide, NaOH, and sodium sulfide, Na.sub.2 S. In an aqueous medium these chemicals hydrolyze based upon the following reactions EQU NaOH+H.sub.2 O6Na++OH--+H.sub.2 O EQU Na.sub.2 S+H.sub.2 O62Na++OH--+HS--
The resulting active ions that are significant to kraft cooking are the hydroxyl ions, OH.sup.-, and the hydrosulfide ions, HS.sup.-. The actual role of these ions are quite different. The hydroxyl ion attacks both the cellulose components of the wood and the lignin. It is believed that the hydrosulfide enhances the hydroxyl ions reaction with the lignin to improve lignin removal, or delignification.
During the cooking process, especially continuous processes, the concentration of hydroxyl ions, or effective alkali (EA), is reduced as the cooking process proceeds. That is, the hydroxyl ions are consumed during the pulping process while the hydrosulfide ion is essentially unaffected.
In the early 1980's, in studies performed at the Swedish Royal Institute of Stockholm (STFI), Sjoblom and others showed that the presence of high concentrations of the hydrosulfide ion in the early stage of kraft cooking improved the resulting yield of the cook. Since that time, efforts have been made to increase the concentration of the hydrosulfide ion, or the sulfidity, of the cooking liquor by chemical addition or manipulation of the recovery process. Examples of such efforts are illustrated in co-pending U.S. application Ser. No. 07/918,855 filed Jul. 27,1992 (Attorney Docket 30-199). The invention takes a much different approach. According to the invention there is provided a new process by which sulfide ion concentration and sulfidity can be enhanced without resorting to chemical addition or manipulation of recovery processes. The invention increases sulfide ion concentration and sulfidity at selected points in a digester by simply manipulating liquor flows.
According to a first aspect of the present invention a method of selectively increasing both the sulfidity and sulfide ion concentration of kraft cooking liquor during kraft cooking of comminuted cellulosic fibrous material is provided. The method comprises the steps of continuously: (a) In a first treatment zone in which impregnation or kraft cooking of comminuted cellulosic fibrous material takes place, causing the material in a slurry of kraft cooking liquor having a first sulfide ion concentration and sulfidity to flow in a first direction through the first zone, from the beginning of the first zone to the end of the first zone. (b) Extracting black liquor from the material at some point after the first treatment zone. (c) Also at some point after the first treatment zone, withdrawing liquid from the material, and adding dilution liquid to the withdrawn liquid, and re-introducing the withdrawn liquid with dilution liquid to the material. And, (d) in a second treatment zone after the first zone subjecting the material to a second kraft cooking liquor having a second sulfide ion concentration and sulfidity greater than the first sulfide ion concentration and sulfidity, including by manipulating and controlling the flow rate of extraction in step (b) and the flow rates of withdrawal of liquid and addition of dilution liquid in step (c).
In the method as described above, steps (b) through (d) are typically practiced so that the second sulfide ion concentration and sulfidity are at least about 20% greater than the first sulfide ion concentration and sulfidity, typically about 20-50% greater, and preferably about 30-40% greater. Also during the practice of step (c) desirably at least half of the dissolved organics are removed from the withdrawn liquor (e.g. by ultra-filtration) prior to re-introduction.
The first zone may be an impregnation zone of a continuous digester or in an impregnation vessel connected to a continuous digester. The first zone may be a vertical co-current cooking or impregnation zone above an extraction screen in a vertical continuous digester. Step (c) may then be practiced so that the reintroduced liquid flows primarily countercurrent to cellulosic material in a second zone in the vertical continuous digester, below the first zone; or step (c) may be practiced to reintroduce the liquid adjacent the beginning of a second co-current zone just below the extraction screen in the vertical continuous digester.
According to another aspect of the present invention a method of increasing the sulfide ion concentration and sulfidity of kraft cooking liquor during kraft cooking of comminuted cellulosic fibrous material comprises the following continuous steps: (a) In a first treatment zone in which impregnation or kraft cooking of comminuted cellulosic fibrous material takes place, causing the material in a slurry of kraft cooking liquor having a first sulfide ion concentration and sulfidity to flow in a first direction through the first zone, from the beginning of the first zone to the end of the first zone. (b) At the end of the first zone removing a substantial amount of the cooking liquor. (c) In a second zone, following the first zone, causing the material to flow counter-currently to the flow of cooking liquor. And, (d) at the beginning of the second zone introducing the material to a second cooking liquor having a higher (e.g. about 20-50%, preferably about 30-40%) sulfide ion concentration and sulfidity than the first liquor.
In a continuous digester the comminuted cellulosic material (chips) flow as a uniform "plug" within the digester. The expression "chip column movement" is often used to describe this flow. This preferred plug flow provides a relatively uniform matrix through which cooking liquor and wash liquor can pass. Although not common, operating conditions which deviate from the design conditions for a digester can cause non-uniformities or discontinuities in this chip matrix which may create areas in which liquor flow may not be uniform. Dislocations or breaks in the chip matrix may create areas in which liquor flow may not be uniform. Dislocations or breaks in the chip column may provide areas where liquor is not distributed uniformly and may result in liquor "channeling". Chips may also channel. Unstable chip columns may have areas where chip movement is not uniform. Chips may move faster in one region than in another.
When chip or liquor movement deviates from the ideal flow, non-uniformities in the cooking process and in the washing process may occur. White liquor which channels can preferentially cook chips adjacent to the channel while other chips are left partially cooked or undercooked. Wash liquor that channels decreases the washing efficiency and results in increased carry-over of dissolved solids and cooking chemicals to the downstream process.
Another aspect of the chip column that affects the uniformity of the cooking and washing process is the chip column "compaction". The weight of the chips and liquor above a section of chips ideally, uniformly compresses the chips so that uniform resistance to liquor flow occurs. If the chip column is not uniform, for example, if the chips are restrained by liquor flow out an extraction screen, i.e., "the hung digester", the chip compaction beneath the screen may be less than that further away from the screen. These areas of reduced compaction may provide regions of reduced resistance to liquor flow and promote liquor channeling.
The introduction of cooking or wash liquor at various locations in the digester may affect the desired uniformity of the chip column. In some situations, fluctuations in this introduction of liquor may further exacerbate the impact this liquor can have on the chip column uniformity and movement.
One liquor source to the digester is the wash filtrate introduction which is also used for pressure control (i.e., "PV-11" in conventional continuous hydraulic digesters, including MCC.RTM. and EMCC.RTM. digesters available from Kamyr, Inc.). The pressure within the digester is controlled by a closed-loop control to a specified value, typically 130-170 psi (e.g. about 165 psi). The pressure within the digester varies due to the amount of chips and liquor fed to the top of the digester, the amount of pulp blown from the digester, the amount of extraction flow removed, the amount of wash filtrate flow added, and other variables. The conventional preferred method of controlling the pressure is to increase or decrease the flow of liquor through valve PV-11. PV-11 is typically located below the wash screens in a Kamyre.RTM. digester and supplies pressurized wash liquor (i.e., "cold blow" liquor) from the downstream brownstock washers.
In some digesters, the vessel pressure is controlled by varying the extraction flow out of the vessel, but this is not a preferred method. Conventionally, the "extraction flow" out of a continuous digester is removed by a screen assembly located in the cooking zone or shortly thereafter. However, since the cellulose material has received some form of heating and/or cooking at this point in the process, the cellulose, for example, softwood chips, are "softer" or more pliable at this point in the treatment. Varying the liquor flow to this region having softer chips to effect pressure control in the vessel can cause undesirable variations in the uniformity of the chip and liquor flow, that is channeling, or screen plugging. Channeling and screen pluggage can lead to non-uniform treatment of the cellulose. The present invention avoids these problems by limiting the variation of liquor removal to areas in the digester where the chips are not as soft and not as sensitive to variations in liquor removal.
As noted previously, the fluctuation in PV-11 flow increases the potential to produce non-uniform, unstable chip movement and liquor flow. In particular, these non-uniformities are promoted in an area that is critical to the efficiency of the counter-current washing/cooking zone directly above. Fluctuations in PV-11 flow increase the potential to produce liquor channeling, non-uniform chip column movement and non-uniform compaction of the chip column.
Another prior art method of controlling the pressure in a continuous digester is the intermittent release of liquor from a cooking circulation to the flash tanks when excessive pressure occurs in the vessel. Typically, a valve, identified in the art as the "PV-10" valve, is located in a cooking circulation of a Kamyr.RTM. continuous digester. See, for example, FIGS. 9-36 of The Pulping of Wood, edited by MacDonald, et al. (1969). When excess pressure develops in the vessel, that is, beyond the control of the pressure controlling PV-11 valve, for example, the PV-10 valve is opened, typically automatically, to direct liquor from the cooking circulation to the flash tanks to relieve the pressure in the vessel. This release of pressure by the PV-10 avoids the activation of electronic interlocks which shut down the digester, typically by shutting down the high-pressure pumps which feed liquor to the digester. This release of pressure by the PV-10 or similar valves is clearly an intermittent recourse to address high pressures in the digester; it is by no means a method of controlling the pressure in the digester on a continuous basis during normal operation.
According to a second aspect of the present invention, the pressure within a digester is controlled in a simple manner which avoids the problems of the control techniques described above, and in fact results in no disruptions of the column of pulp continuously moving downwardly in the digester anywhere within the digester. According to this aspect of the invention, a method of controlling the pressure of a vertical continuous comminuted cellulosic fibrous material digester, a main extraction, and at least one additional extraction-dilution loop distinct from the main extraction is provided, comprising the step of: (a) Withdrawing liquor from, and introducing liquor into, the digester at the at least one additional extraction-dilution loop to maintain the pressure in the digester at a desired superatmospheric level while avoiding non-uniform, unstable material movement in the countercurrent washing zone. Step (a) is typically practiced to maintain the pressure in the digester at about 130-170 psi (e.g. about 165 psi).
The digester also typically comprises a wash dilution liquid introduction mechanism below the wash screens. In this case there is preferably also the further step (b) of controlling the pressure in the vessel by also, in addition to step (a), controlling the amount of wash dilution liquid introduced into the digester by the wash dilution liquid introduction mechanism (e.g. PV-11). There may also be the further step (c), in addition to step (a), or in addition to steps (a) and (b), of controlling the pressure in the vessel by also varying the extraction flow out of the digester through the main extraction. Alternatively, the control of pressure in the digester, by manipulating liquid extractions and introductions, may consist of (that is be provided only by) the practice of step (a), although still there will be other variables which can control the pressure including the amount of chips and liquor fed to the top of the digester, etc., as described above.
At least two additional extraction-dilution loops may be provided, in which case step (a) may be practiced by varying the liquid flow into and out of the digester using at least two different extraction-dilution loops. The volume and location for introduction of pressure controlling liquid can be controlled to least-affect the column movement in the digester. The optimum volume and location will vary from digester to digester, depending upon which area in the digester has the most stable column movement. However in all cases the significant potential source of non-uniform liquor distribution and non-uniform column movement in the critical counter-current washing/cooking zone is minimized or eliminated.
According to another aspect of the present invention, a method of controlling the pressure of a vertical continuous digester is provided comprising the steps of: (a) withdrawing liquor from, and introducing liquor into, the digester at the at least one additional extraction-dilution loop to maintain the pressure in the digester at a desired superatmospheric level; and (b) controlling the pressure in the vessel by also, in addition to step (a), controlling the amount of wash dilution liquid introduced into the digester by the wash dilution liquid introduction mechanism; or (c) controlling the pressure in the vessel by also, in addition to step (a), varying the extraction flow out of the digester; step (a), and at least one of steps (b) and (c), being practiced to avoid disruptions of a column of pulp continuously moving downwardly in the digester anywhere in the digester.
Of course the selective sulfide ion concentration and sulfidity increasing aspect of the invention may be combined with the continuous digester pressure control aspect of the invention, so that the advantages of both are obtained in a continuous digester, and they both can be obtained at the same time utilizing the same circulation/extraction-dilution loop or loops.
According to the preferred aspect of the present invention, a method of controlling the pressure of a vertical continuous comminuted cellulosic fibrous material digester is provided that is advantageous because it varies the liquor flow to the digester at a point where the effect upon column movement and treatment is minimized. Specifically, this point in the cooking process occurs before the chips are exposed to cooking temperature and become "softer", that is, more pliable and prone to compaction. This softer, more pliable chip mass is more sensitive to variations in liquor movement that can result in stagnation or channeling of chips and liquor or both. These softer, more pliable chip masses typically occur in the latter stages of cooking, toward the bottom of the digester. Also, since changes to the flow patterns in the bottom of the digester can affect the flow patterns throughout the height of the digester, it is preferred to avoid any variations in liquor flows to the lower part of the digester. This lower part of the digester may be a counter-current or a co-current zone, it may be a cooking or a washing zone. The digester includes a main extraction, a treatment zone, a pressure-control extraction (e.g. in a zone--at an upper part of the digester--relatively insensitive to changes in the flow rate of liquid introduction or removal), and optionally at least one recirculation-dilution loop distinct from the main extraction and the pressure-control extraction. The method comprises: (a) Controlling the pressure in the digester primarily by varying the flow rate of liquor extracted from the pressure-control extraction to maintain the pressure in the digester at a desired superatmospheric level while avoiding non-uniform, unstable material and liquid movement in the treatment zone. There may also be (b) introducing dilution liquor into the digester at the at least one recirculation-dilution loop.
According to the present invention a liquor is extracted to control pressure early in the treatment having an effective alkali (EA) which is lower than the EA of the liquor removed later in the treatment. Typically, prior art systems for controlling pressure in a digester, such as the system shown by MacDonald, extract liquor during the cooking stage or after the cooking stage having an EA greater than 15 grams per liter (g/l) as NaOH. According to the present invention, the EA of the liquor removed is typically less than 15 g/l, preferably less than 10 g/l. Among other things, the removal of liquor having a lower EA compared to one having a higher EA does not waste cooking chemical.
Also, according to the present invention, liquor is preferably removed from the chips at a location prior to the cellulose material being heated, for example, to cooking temperature. That is, according to the present invention, pressure is controlled in the vessel by removing liquor prior to cellulose material and liquor being heated to a temperature of 150.degree. C., typically 140.degree. C., and preferably before it is heated to 130.degree. C. Controlling the pressure by removing the liquor prior to heating has the advantages of removing wood moisture prior to heating to avoid wasting energy heating the wood moisture and also avoiding the thermal inefficiency of removing liquor after the liquor has been heated.
Furthermore, the control of pressure within a digester by extraction in the upper part of the vessel is not limited to digesters having a dilution-extraction stage. The broadest embodiment includes a digester having a first extraction stage below or during the cooking process and a second pressure-controlling extraction above or before the cooking process. The lower extraction is preferably held relatively constant, the upper extraction is varied based upon vessel pressure.
In the method (a) may be practiced by substantially exclusively (that is except for during excessive over pressure or under pressure conditions) by varying the flow rate of liquor extracted from the pressure-control extraction. Usually (a) is practiced by controlling the amount of flow through a flow control valve, such as by automatically controlling the flow control valve in response to pressure sensed by at least one pressure sensor. The practice of (a) is typically effected to control the pressure in the digester to be at a predetermined level within the range of 130-170 psi gage. Substantially only during excessive over pressure and under pressure conditions, (a) is also practiced by controlling the rate of extraction or dilution from or to at least one of the at least one recirculation-dilution loop and the main extraction. That is substantially except during excessive over pressure and under pressure conditions the extraction flow from and dilution flow into the at least one recirculation-dilution loop and the main extraction are maintained substantially constant. It is understood that the flow of dilution and extraction liquor will vary with the rate of production. That is, the nominal flow of dilution and extraction, including the main extraction, may vary with changes in the production rate.
Cold blow filtrate and washer filtrate are preferred as the dilution liquors in practicing (b), although other dilution liquors may be used. Also preferably the pressure-control extraction is substantially the first extraction of the digester, and (a) is practiced above or before the main extraction and above or before the at least one recirculation-dilution loop, preferably where a co-current zone is provided thereabove.
It is the primary object of the present invention to increase the effectiveness and practicality of kraft cooking of comminuted cellulosic fibrous material in the production of cellulosic (paper) pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.